The invention relates to novel adhesion promoters which are highly active in mixtures of elastomers and mineral fillers and are adapted for crosslinking by sulfur or peroxide. The novel adhesion promoters consist of hydrosilylated polybutadiene oils of a specific microstructure.
Carbon blacks of various specifications are used as one component of elastomer mixtures. Addition of carbon black is done less for rendering the vulcanizates produced from this mixture more economical than for improving the characteristics of the vulcanizates, especially tear strength, modulus, hardness, tear propagation resistance and abrasion resistance. Therefore, carbon black is considered an "active" or "reinforcing" filler.
For various reasons, the utilization of carbon black in elastomer mixtures is limited. Mixtures prepared therewith are necessarily black, rather than colored or white. Good quality carbon blacks have become so expensive that it has become increasingly desirable to replace the carbon blacks by inexpensive mineral fillers, such as silicic acid, kaolin, aluminum hydroxide and glass.
Such substitution also reduces the proportion of components derived from crude oil, which raises critical questions as to availability. Utilization of highly active silicic acids is advantageous for optimizing certain properties, e.g., notch impact strength.
Light-colored mineral fillers have been used heretofore, primarily for purely economical reasons. However, considerable losses of properties required for practical applications for example, heat buildup, elasticity and compression set, had to be tolerated.
These disadvantages can be at least partially eliminated by the use of so-called adhesion promoters, which are substances having a certain affinity to the filler and to the elastomer. This affinity is preferably manifested by chemical reactivity toward both substrates.
Organofunctional silanes are known as tackifiers. These compounds are of the formula R--SiX.sub.3, wherein X in most cases is alkoxy, or less commonly halogen, and R is alkyl or aryl substituted by a functional group. Examples of such abrasion promoters derived from organofunctional, low-molecular silanes used in elastomer technology with particular frequency and which are offered commercially by several manufacturers are vinyl tris(methoxyethoxy)silane and .gamma.-mercaptopropyltriethoxysilane. Although these products yield satisfactory results with respect to the properties of elastomer-filler combinations produced therewith, each has several disadvantages in use.
Each is usable with optimal results only for a specific type of crosslinking technique. The mercaptosilane and related compounds, e.g., the polysulfide derivable therefrom, of U.S. Pat. Nos. 3,873,489, 3,842,111 and 3,978,103, is suitable only for conventional sulfur vulcanization, whereas the vinylsilane is suitable only for peroxide vulcanization. The mercaptosilane has an unpleasant odor and a tendency toward premature vulcanization or scorch of mixtures containing this compound. These organofunctional silanes are extraordinarily expensive compared to other components of the elastomer mixture and have toxicity upon inhalation and skin contact. This toxicity cannot be ignored.
Numerous attempts have been made to synthesize similarly effective polymeric adhesion promoters. Natural rubber and SBR can be hydrosilylated by heating with trichlorosilane to about 300.degree. C. (U.S. Pat. No. 2,475,122). The resulting reaction products adhere well to sheet glass (U.S. Pat. No. 2,557,778).
Photochemical hydrosilylation of a liquid polybutadiene obtained by anionic polymerization is described in U.S. Pat. No. 2,952,576, which is directed to glass fibers, coated with this material, for the reinforcement of unsaturated polyester resins. Although the microstructure of the liquid polybutadiene employed is not disclosed, the compound is produced using a sodium suspension and is thought, from comparison with literature data, to contain about 60-70% vinyl, 30-20% trans-vinylene, and about 10% cis-vinylene.
Hydrosilylation of polybutadienes by platinum containing-catalysts is described in U.S. Pat. No. 3,470,226 and DT-OS No. 1,720,527. The materials are intermediates in the manufacture of foam stabilizers and/or laminating resins. These references do not suggest using the reaction products in vulcanizable elastomer-filler mixtures. Furthermore, both products have a high vinyl content whereas the residual double bonds are predominantly of trans-vinylene. Polybutadienes of this microstructure, even at relatively low molecular weights, have very high viscosity at room temperature. Handling, dosing, and mixing is therefore extraordinarily difficult. The same limitations apply to the hydrosilylated derivatives.
Conventional Pt catalysis for hydrosilylation is also utilized in U.S. Pat. No. 3,759,869, directed to polymers having molecular weights between 500 and 50,000, which contain at least 25% of the structure ##STR2##
This corresponds, using pure polybutadiene as the basic polymer, to the attachment of a reactive silyl group --SiX.sub.3 to approximately each tenth monomer unit. The examples disclose hydrosilylation of a polybutadiene having an average molecular weight of 1,000 and a vinyl content of 90%, based on total double bonds, with practically 100% saturation of all vinyl groups present. Mixtures of such products and/or of their derivatives, obtained by secondary reactions, with low-molecular polypropylene (molecular weight 5,000) and/or EPM rubber are merely mentioned without any statements regarding their effectiveness. Additionally, such essentially saturated polybutadiene derivatives and/or fillers provided therewith are scarcely suitable, based on the lack of double bonds, for linkage with a polymer network formed by sulfur or peroxide vulcanization.
U.S. Pat. No. 3,881,536 claims hydrosilylation of elastomeric polymers containing at least preferably 5-30% by weight of vinyl groups, and the use thereof as coupling agents for the vulcanization of a vulcanizable elastomer containing a silicic-acid-containing pigment. These products can, owing to high molecular weight, be used only in solution.
Other references on hydrosilylated hydrocarbon polymers include Streck et al, U.S. Pat. Nos. 3,920,714, 3,920,715 and 3,929,850, the disclosures of which are herein incorporated by reference.
It is therefore an object of the present invention to provide an adhesion promoter effective in elastomer-filler mixtures crosslinked by a peroxide or by sulfur, which does not exhibit the disadvantages of the above products.